The skin is continuously exposed to changes in the external environment, including oxidative insults, heat, cold, UV radiation, injury, and mechanical stresses. The stratum corneum, composed of terminally differentiated keratinocytes, constitutes the natural barrier that prevents loss of water and prevents entry of infectious agents (e.g., bacteria, viruses), small objects (e.g., particles), and a broad variety of water-soluble chemicals.
Intermediate filaments (IFs), microtubules (MTs) and microfilaments (MFs) constitute the cytoskeleton and play important roles in the organization and mechanical Integrity of skin keratinocytes (Fuchs and Cleveland, 1998). Keratins are a large family of proteins that form the intermediate filament cytokeleton in epithelial cells. Keratins are encoded by two groups of genes, type I and II, which are distinct at the level of genomic structure and nucleotide sequence. Each type of keratin gene is clustered within separate loci in the mouse and human genomes (Fuchs, 1995; Schweizer at al., 2006). Type II keratin proteins, which include K1-K8/K71-K74 in soft epithelia and K81-K86 in hard epithelia, such as hair, nail, and oral papilla, are larger (52 to 70 kDa) and basic-neutral in charge; type I keratins, which comprise K9-K28 in soft epithelia and K31-K40 in hard epithelia, are smaller (40 to 64 kDa) and acidic (Schweizer et al., 2006). The physical proximity, identical substructure and transcriptional orientation, and high sequence homology of type II keratin genes K5, K6α, K6β and K6hf, and type I keratin genes K14, K16, K17 and K17n, strongly suggest that each subset was generated through successive duplications from a common ancestral gone (Wong at al., 2005). Type I epidermal keratin genes K14, K16 and K17 share high amino acid sequence identity (Troyanovsky et al., 1992; McGowan and Coulombe, 1998; 1998b), and are structurally and functionally related (Paladini and Coulombe, 1999; Coulombe et al., 2004; Tong and Coulombe, 2006).
In most epithelial cells the keratin filament network spans the entire cytoplasm, from the surface of the nucleus to the cell periphery, where it contacts cell-matrix (hemidesmosomes) and cell-cell (desmosomes) adhesive sites (e.g., Fuchs, 1995; Gu and Coulombe, 2007). Keratin Intermediate filaments provide cells and tissues with mechanical resilience and protects them against physical stress. Disruption of the keratin scaffold leads to tissue and cell fragility in the skin and its appendages (hair, nail, glands), oral mucosa, and cornea. Several genetic diseases are caused by dominantly-acting mutations altering the coding sequence of keratin proteins (Fuchs and Cleveland, 1998; On and Coulombe, 2007; Irvine and McLean, 1999; Omary et al., 2004). Most of these mutations are missense or small in-frame insertions or deletions affecting the central rod domain of keratin proteins, and interfering with their structural support function (Cassidy et al. 2002; Gu and Coulombe 2007).
Epidermolysis bullosa simplex (BBS) is a rare autosomal dominant disease in which the epidermis loses its integrity following trivial mechanical trauma (Fine at al., 1991; 2000). The disease is characterized by extreme fragility of the keratinocytes, and skin blistering, resulting from missense mutations in the gene that encodes keratin 5 (K5) or keratin 14 (K14) (Fuchs and Cleveland, 1998; Cassidy at al. 2002; Gu and Coulombs 2007; Omary et al., 2004). K5 and K14, which are abundant cellular proteins, normally co-polymerize to form an intricate network of 10-12 nm-wide, “intermediate-sized” filaments in basal keratinocytes of epidermis and related epithelia (Nelson and Sun, 1983; Fuchs, 1995). EBS may manifest itself as a relatively mild blistering condition involving the hands and feet (EBS, Weber-Cockayne type), or as a generalized blistering condition, sometimes associated with mucosal blistering that involves the oropharynx, the esophagus and ocular mucosa, and which can be fatal (e.g., EBS, Dowling-Meara type). In individuals affected by EBS Weber-Cockayne (EBS-WC), blisters are rarely present at birth and may occur on the knees and shins with crawling, or on the feet in late infancy or later, during adolescence or early adulthood. Neonates affected by EBS, Koebner type (EBS-K), present blisters at birth or develop blisters within the first few months of life (Fine at al., 1991; 2000). In individuals suffering from EBS with mottled pigmentation (EBS-MP), skin fragility is evident at birth and children develop progressive brown pigmentation over time, interspersed with depigmented spots on the trunk and extremities, which disappears in adult life (see Gu and Coulombs, 2007, and refs. therein). Individuals affected by BBS-DM develop widespread and severe blistering and/or multiple grouped clumps of small blisters at birth, with hyperkeratosis of the palms and soles, that improves during mid to late childhood. The blistering in EBS-DM can be severe enough to result in neonatal or infant death (Fine at al., 2000).
Management of all types of EBS consists of supportive care to protect the skin from blistering, dressings that promotes healing, and prevention and treatment of secondary infection. These treatment options are therefore palliative and have limited success. Furthermore, EBS is representative of a large number of tissue fragility conditions caused by inherited mutations in intermediate filament protein-encoding genes (sue Fuchs and Cleveland, 1998; Cassidy at al., 2002; Omary et al., 2004; Gu and Coulombs, 2007).
Accordingly, there is a need in the art for improved treatment options for EBS and the present invention satisfies that need.